Process for producing polyester sheet and film

ABSTRACT

A process for producing a polyester sheet by dropping a molten polyester sheet extruded from an orifice-form nozzle on a cooling roll having the grooves of a large number of micro-cracks formed on the surface, closely adhering it to the cooling roll and solidifying it on the cooling roll, wherein 
     the surface temperature (T, ° C.) of the molten polyester sheet 10 mm below the orifice-form nozzle is maintained at a temperature which satisfies the following expression (1): 
     
       
         ( Tc +20)° C.≦ T ≦( Tm +40)° C.  (1)  
       
     
     wherein Tc and Tm are the falling temperature crystallization temperature (° C.) and melting point (° C.) of the polyester, respectively and T is as defined hereinabove, and the surface temperature of the cooling roll when it contacts the molten polyester sheet is controlled to a range of 5 to 100° C. to continuously form the polyester sheet while preventing the adhesion of a sublimate from the molten polyester to the inside of the groove of each micro-crack of the cooling roll.

FIELD OF THE INVENTION

The present invention relates to polyester sheet and film productionprocesses. More specifically, it relates to a polyester sheet productionprocess which can suppress the adhesion of a low-molecular weightsublimate to the inside of a micro-crack when a polyester sheet isextrusion molded using a cooling roll having micro-cracks formed on thesurface and which is capable of producing a high-quality sheet havingexcellent smoothness stably at a high speed for a long time and to aprocess for producing a biaxially oriented polyester film from theobtained sheet.

PRIOR ART

As means of casting a polymer sheet, there has been known a method inwhich a sheet product of a molten polymer extruded from an orifice-formnozzle is closely adhered to the surface of a cooling roll to besolidified by an electrostatic adhesion method or gas pressure method. Acooling roll having a smooth surface (mirror finished surface) isgenerally used as the cooling roll in this method, and air caught in agap between the cooling roll surface and the sheet product must beremoved when the sheet product of the molten polymer is closely adheredto the surface of the cooling roll.

The removal of the caught air becomes more difficult as the castingspeed increases, thereby causing various problems. For instance, sincethe caught air is existent in the form of a bubble in the electrostaticadhesion method, it causes a sheet surface defect, resulting indeterioration in the smoothness of the sheet. In the gas pressuremethod, the caught air causes insufficient heat transmission between thesheet and the cooling roll with the result that the sheet is not cooledenough. When the sheet is not cooled enough, the sublimation of alow-molecular weight compound contained in the molten polyestercontinues whereby the accumulation of the low-molecular weight compoundon the surface of the cooling roll becomes marked and the compound istransferred to the surface of the sheet, thereby causing an orangeskin-like defect.

When the amount of the low-molecular weight compound accumulated on thesurface of the cooling roll increases, insufficient heat transmissionbecomes more marked, thereby making high-speed casting impossible.

This problem can be improved by exchanging the cooling roll proposed byJP-A 62-196118 (the term “JP-A” as used herein means an “unexaminedpublished Japanese patent application”) for a cooling roll having amicro-crack formed surface (micro-cracked surface). That is, since thecaught air is scattered through the grooves of the micro-cracks in theelectrostatic adhesion method, a bubble-form defect can be improved andthe casting speed can be greatly increased. Since the caught air is alsoscattered through the grooves of the micro-cracks in the gas pressuremethod, a reduction in the heat transmission speed between the sheet andthe cooling roll can be prevented and the occurrence of an orangeskin-like defect can also be avoided.

However, a process for extrusion molding a polyester sheet using acooling roll having a micro-cracked surface involves a new problem thatthe ventilation resistance of the micro-crack increases along thepassage of time, thereby reducing the function of scattering the caughtair through the grooves of the micro-crack in a short period of time.The cause of this is that a low-molecular weight compound sublimatedfrom a sheet product is accumulated in the inside of the groove of themicro-crack, thereby clogging the groove. Therefore, the low-molecularweight compound must be removed frequently from the groove, which posesa production problem.

To remove the deposit accumulated on the surface of the cooling roll,there are known (1) a method in which a non-contact portion between thesurface of a cooling roll and a polyester sheet is always cleaned bywater or a solvent, and the water or solvent is dried and sucked to beremoved as disclosed by JP-B 47-3917 and JP-B 48-4465 (the term “JP-B”as used herein means an “examined Japanese patent publication”), (2) amethod in which the surface of a cooling roll is subjected to a coronatreatment as disclosed by JP-A 57-51426 and (3) a method in which adeposit is decomposed and removed by irradiating the surface of acooling roll with ultraviolet radiation as disclosed by JP-B 3-65775.

However, the above method (1) is effective for a cooling roll having amirror finished surface but cannot be applied to a cooling roll having amicro-cracked surface because it is difficult to wash the inside of thegroove and remove a liquid in the inside of the groove. The methods (2)and (3) are also effective for a cooling roll having a mirror finishedsurface but cannot be applied to a cooling roll having a micro-crackedsurface because they have a poor effect of decomposing and removing thedeposit in the inside of the groove.

In order to wash the inside of the groove of each micro-crack of thecooling roll, JP-A 10-217307 discloses a method in which the coolingroll is immersed in a chemical bath. However, this method involvesproblems to be solved for practical application, such as theinterruption of sheet production and the complicated operation ofimmersing the cooling roll in a chemical bath.

Other solutions to the above problems include a method in which thewidth of groove of the micro-crack is increased. However, when the widthof the groove is increased, new problems are expected to arise that thetransfer of the micro-crack to the sheet may cause an orange skin-likedefect and it is extremely difficult to form micro-cracks having alarger width uniformly on the surface of the cooling roll.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a polyester sheetproduction process which can reduce the frequency of interrupting sheetproduction by cooling a polyester sheet while the increase speed ofventilation resistance caused by the clogging of the groove of eachmicro-crack is controlled to a low level and which eliminates the needfor bulky equipment.

It is another object of the present invention to provide a polyestersheet production process which can provide a high-quality film withouttransfer by using a cooling roll having the grooves of micro-cracks.

It is still another object of the present invention to provide a processfor producing a biaxially oriented polyester film from a polyester sheetproduced by the above process of the present invention.

Other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, firstly, the above objects andadvantages of the present invention are attained by a process forproducing a polyester sheet by dropping a molten polyester sheetextruded from an orifice-form nozzle on a cooling roll having thegrooves of a large number of micro-cracks formed on the surface, closelyadhering it to the cooling roll and solidifying it on the cooling roll,wherein

the surface temperature (T, ° C.) of the molten polyester sheet 10 mmbelow the orifice-form nozzle is maintained at a temperature whichsatisfies the following expression (1):

(Tc+20)° C.≦T≦(Tm+40)° C.  (1)

wherein Tc and Tm are the falling temperature crystallizationtemperature (° C.) and melting point (° C.) of the polyester,respectively and T is as defined hereinabove, and the surfacetemperature of the cooling roll when it contacts the molten polyestersheet is controlled to a range of 5 to (Tg−20)° C. (Tg is the glasstransition temperature of the polyester) to continuously form thepolyester sheet while preventing the adhesion of a sublimate from themolten polyester to the inside of the groove of each micro-crack of thecooling roll.

According to the present invention, secondly, the above objects andadvantages of the present invention are attained by a process forproducing a biaxially oriented polyester film, comprising the step ofbiaxially orienting the polyester sheet obtained by the above process ofthe preset invention in a longitudinal direction and a transversedirection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an instrument for measuring the ventilationresistance of the groove of each micro-crack of a cooling roll; and

FIG. 2 is an enlarged sectional view of a sucker portion of theventilation resistance measuring instrument of FIG. 1.

THE PREFERRED EMBODIMENTS OF THE INVENTION

The polyester sheet production process of the present invention will befirst described hereinbelow.

In the present invention, a cooling roll having the grooves of a largenumber of micro-cracks formed on the surface is used as the coolingroll. That is, a large number of fine and irregular grooves are existenton the flat surface of the cooling roll and form micro-cracks from thesurface to a deep portion.

The grooves of the micro-cracks which are formed irregularly asdescribed above have a ventilation resistance measured by a vacuumleakage method to be descried hereinafter of preferably 10,000 sec orless, more preferably 5,000 sec or less, particularly preferably 1,000sec or less. The most preferred ventilation resistance is in the rangeof 2 to 500 sec. When the ventilation resistance exceeds 10,000 sec, theimprovement of the casting speed is hardly expected.

The ventilation resistance in the present invention is represented by atime required for a reduction in the degree of vacuum from a certainvalue to another certain value when a vacuum region is formed in thesurface of the cooling roll, air flows into the region through thegrooves of the micro-cracks on the surface of the cooling roll and thisvacuum suction is stopped. As for a specific method of measuringventilation resistance, as shown in the schematic diagram of FIG. 1, avacuum pump 14 is connected to one end of a vessel 11 equipped with avacuum gauge 12 through a vacuum cock 13 and a rubber sucker 16 (forexample, FPM. PFYK-40 of Myotoku Co., Ltd.) is attached to the other endby a vacuum hose 15. The effective volume from the vacuum cock 13 to thesucker 16 is 100 cc. As shown in the enlarged sectional view of thesucker portion of FIG. 2, a 40 mm-diameter sucker (16, 22) is pressedagainst the surface 24 of the cooling roll and a 30 mm-diameter poroussheet (8-L-500 Naslon low-density sintered material of Nippon SeisenCo., Ltd.) 23 is placed and pressed against the center of the sucker sothat it contacts the outer surface of the sucker. When the inside of the100 cc vessel is evacuated to −93.1 kPa (−700 mmHg) or less by thevacuum pump 14 and the cock 13 is closed, air flows into the vacuumsystem through the grooves of the surface surrounded by the sucker,resulting in a reduction in the degree of vacuum. The time required fora reduction in the degree of vacuum from −93.1 kPa to −86.45 kPa (−700mmHg to −650 mmHg) is defined as ventilation resistance. Before themeasurement of ventilation resistance, it is confirmed that theventilation resistance of a polished glass plate is 20,000 sec or moreto check the vacuum leakage of the measuring instrument.

Preferably, the cooling roll has the grooves of at least fivemicro-cracks intersecting a 10 mm long virtual straight line drawn onthe surface of the cooling roll in any direction, 70% or more of thegrooves of the intersecting micro-cracks have a width of 0.1 to 100 μmat intersections with the virtual straight line, and the total width ofall the grooves of the micro-cracks intersecting the virtual straightline at the intersections is 5 mm or less. When the molten sheet isclosely adhered to the surface of the cooling roll, air caught betweenthem is exhausted through the grooves of the micro-cracks.

Since the transfer intensity of the grooves of the micro-cracks to thesheet (orange skin-like defect) depends on the width of the groove ofeach micro-crack, the width of the groove is preferably set to 100 μm orless, more preferably 50 μm or less, particularly desirably 20 μm orless to weaken the transfer intensity.

Further, the total opening area of the grooves of a large number ofmicro-cracks open to the surface of the cooling roll is preferably 0.01to 0.3 mm² per 1 mm² of the surface of the cooling roll.

When this value is smaller than 0.01 mm², it is difficult to increasethe casting speed to the full and when the value is larger than 0.3 mm²,it is difficult to release the polyester sheet solidified on the coolingroll from the cooling roll at the time of casting.

The lower limit value is preferably 0.02 mm² to increase the castingspeed and the upper limit value is preferably 0.2 mm² to release thepolyester sheet from the cooling roll smoothly.

The total opening area of the grooves may be obtained by measuring atleast 1 mm² portion of the surface in contact with the molten polyestersheet of the cooling roll. However, it is preferably obtained bymeasuring at least four locations of the surface in contact with themolten polyester sheet of the cooling roll (4 locations crossing oneanother at an angle of 90° in a circumferential direction of a centerportion in the width direction of the cooling roll) and averaging themeasurement values, particularly preferably by measuring 12 locations(12 locations crossing one another at an angle of 90° in acircumferential direction of a center portion in the width direction ofthe cooling roll and intermediate portions between both end portions andthe center portion) and averaging the measurement values.

In the process of the present invention, the molten polyester sheetextruded from the orifice-form nozzle is dropped on the surface of theabove cooling roll, closely adhered to the cooling roll and furthersolidified on the cooling roll.

The orifice-form nozzle is a nozzle having a linear opening, such as a Tdie, fish-tail die or I die. A nozzle incorporating a pipe in parallelto an orifice in a width direction at the end of the nozzle isadvantageous.

As means of closely adhering the molten polyester sheet to the surfaceof the cooling roll, preferred are, for example, an electrostaticadhesion method in which a molten polyester sheet is closely adhered tothe surface of a cooling roll with Coulomb force by applyingelectrostatic charge to the sheet and a gas pressure method in which amolten polyester sheet is closely adhered to the surface of a coolingroll by applying the static pressure of gas essentially composed of airto the sheet.

Various methods of cooling the cooling roll may be employed.

A cooling roll for cooling a polyester sheet on the surface thereof byintroducing cooling water therein and discharging it is preferred. Inthis case, it is preferred to control the temperature of the coolingwater to be discharged to a temperature 1 to 10° C. higher than thetemperature of the cooling water to be introduced. When the temperaturerise is more than 10° C., the cooling capacity of the cooling rolldecreases, thereby increasing the temperature of the sheet in thecooling step and the amount of a low molecular weight compoundsublimated from the sheet product and making it difficult to release thesheet from the cooling roll as the sheet product sticks to the coolingroll with the result that the molding speed of the sheet must bereduced. In addition, the quality of the obtained sheet varies in thewidth direction due to a temperature difference in the width directionof the cooling roll.

To reduce the temperature rise to 1° C. or less, the amount of coolingwater to be introduced into the cooling roll must be made larger thanrequired, thereby increasing the size of equipment such as a coolingsystem and a pump and boosting equipment cost and running costdisadvantageously.

The upper limit of the temperature rise is preferably 8° C.,particularly preferably 6° C. The lower limit of the temperature rise ispreferably 2° C. to obtain a high-quality sheet, particularly preferably3° C.

The diameter of the cooling roll is preferably in the range of 0.6 to4.0 m.

When the diameter of the cooling roll is smaller than 0.6 m, its coolingcapacity becomes insufficient whereby the grooves of the micro-cracksmay be occluded quickly and it may be difficult to release the sheetfrom the cooling roll. When the diameter of the cooling roll is largerthan 4.0 m, its cooling capacity is sufficient but the roll is too big,thereby making it difficult to surface finish the micro-cracks andincreasing processing costs for that. The lower limit of the diameter ofa practical cooling roll is preferably 0.8 m, particularly preferably1.0 m. The upper limit of the diameter is preferably 3.5 m, particularlypreferably 3.0 m.

The thickness of the shell having the grooves of a large number ofmicro-cracks of the cooling roll is preferably 5 to 30 mm.

When the thickness of the shell of the cooling roll is smaller than 5mm, it is difficult to retain sufficiently the strength of the coolingroll and the flatness of the sheet is easily deteriorated by thedeformation of the roll. When the thickness of the shell is larger than30 mm, heat transmission from the cooling water deteriorates whereby thesheet may not be cooled enough. The lower limit of the thickness of theshell of the practical cooling roll is 7 mm, preferably 9 mm and theupper limit thereof is 25 mm, preferably 20 mm.

In the process of the present invention, the surface temperature (T, °C.) of the molten polyester sheet 10 mm below the orifice-form nozzle ismaintained at a temperature which satisfies the following expression(1):

(Tc+20)° C.≦T≦(Tm+40)° C.  (1)

wherein Tc and Tm are the falling temperature crystallizationtemperature (° C.) and melting point (° C.) of the polyester,respectively.

When the surface temperature of the sheet is higher than (Tm+40)° C.,the effect of the present invention cannot be obtained and when thesurface temperature is lower than (Tc+20)° C., a projection of thesolidified polyester or the like is formed on the orifice at the end ofthe nozzle and contacted to the sheet extruded from the orifice, therebyforming a striped defect on the sheet and the fallen projection isadhered to the sheet to become a sheet defect. The upper limit of thesurface temperature of the sheet is preferably (Tm+30)° C. to make theeffect of the present invention more marked, particularly preferably(Tm+25)° C. The lower limit of the surface temperature of the sheet ispreferably (Tc+25)° C. to achieve the excellent surface smoothness ofthe sheet, particularly preferably (Tc+30)° C.

By maintaining the surface temperature of the sheet at the range of thepresent invention, the low-molecular weight compound contained in themolten polyester is suppressed from diffusing from the interior to thesurface of the sheet and the sublimation of the compound from thesurface is further suppressed to control the accumulation speed of thecompound on the surface of the cooling roll. Further, as not only theaccumulation of the low-molecular weight compound on the surface of thecooling roll but also the accumulation of the compound in the inside ofthe groove of each micro-crack can be suppressed, time changes in theventilation resistance of the groove of the micro-crack can becontrolled, thereby making it possible to maintain the casting speed ata high level for a long time.

The surface temperature can be measured by a radiation type thermometer.The radiation type thermometer can selectively measure the surfacetemperature of a sheet product and does not provide disturbance to thesheet product because of non-contact measurement.

The melting point and falling temperature crystallization temperature ofthe polyester are measured by a differential scanning calorimeter (DSC).About 10 mg of a polyester sample is placed in an aluminum pan andheated at 300° C. for 5 minutes to be molten and the pan is placed onice to be quenched so as to prepare a measurement sample. Thetemperature of this measurement sample is increased from 25° C. at atemperature elevation rate of 20° C./min to take its melting peaktemperature as its melting point (Tm: ° C.). Meanwhile, about 10 mg of apolyester sample is placed in an aluminum pan and heated at 300° C. for5 minutes to be molten and then the temperature is lowered at a rate of10° C./min to take its crystallization peak temperature as its fallingtemperature crystallization temperature (Tc: ° C.).

To maintain the surface temperature of the sheet at the above range, arefrigerant such as air or oil is caused to run in the nozzle or the endof the nozzle to carry out heat exchange. In order to reduce the surfacetemperature of the sheet without causing temperature variations, therefrigerant is circulated by incorporating a pipe in parallel to theorifice in the width direction at the end of the nozzle, or the end ofthe nozzle is controlled by incorporating a heat pipe to eliminatetemperature variations.

In the process of the present invention, the surface temperature of thecooling roll before it contacts the molten polyester sheet is maintainedat a range of 5 to (Tg−20)° C. By maintaining the surface temperature ofthe cooling roll at a range of 5 to (Tg−20)° C., the molten polyestersheet maintained at the above surface temperature T is taken up and theadhesion and accumulation of the sublimate in the groove of eachmicro-crack of the cooling roll can be effectively prevented. Thesurface temperature of the roll is preferably in the range of 10 to(Tg−25)° C., more preferably 15 to (Tg−30)° C.

The polyester in the present invention is preferably an aromaticpolyester comprising an aromatic dicarboxylic acid component and analiphatic glycol component.

The aromatic polyester is preferably a polyester comprising terephthalicacid or 2,6-naphthalenedicarboxylic acid as the main aromaticdicarboxylic acid component.

The polyester comprising terephthalic acid as the main aromaticdicarboxylic acid component comprises terephthalic acid in an amount ofpreferably 50 mol % or more, particularly preferably 80 mol % or morebased on the total of all the dicarboxylic acid components. Dicarboxylicacid components other than terephthalic acid include2,6-naphthalenedicarboxylic acid, isophthalic acid,hexahydroterephthalic acid, 4,4′-diphenyldicarboxylic acid, adipic acid,sebacic acid and dodecanedicarboxylic acid. Examples of the aliphaticglycol component include ethylene glycol, diethylene glycol, propyleneglycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol,1,5-pentanediol, 1,6-hexanediol, cyclohexanedimethanol, polyethyleneglycol and polytetramethylene glycol. Out of these, ethylene glycol,1,3-propanediol and 1,4-butanediol are preferred and ethylene glycol isparticularly preferred because the obtained polyester film has excellentmechanical properties and thermal properties.

The polyester comprising terephthalic acid as the main dicarboxylic acidcomponent is, for example, polyethylene terephthalate. Polyethyleneterephthalate may comprise a component other than terephthalic acid andethylene glycol in an amount of 50 mol % or less, particularly 20 mol %or less.

The polyester comprising 2,6-naphthalenedicarboxylic acid as the mainaromatic dicarboxylic acid comprises 2,6-naphthalenedicarboxylic acid inan amount of preferably 50 mol % or more, particularly preferably 80 mol% or more based on the total of all the dicarboxylic acid componet.Dicarboxylic acid components other than 2,6-naphthalenedicarboxylic acidinclude terephthalic acid, isophthalic acid, hexahydroterephthalic acid,4,4′-diphenyldicarboxylic acid, adipic acid, sebacic acid anddodecanedicarboxylic acid. Examples of the aliphatic glycol componentare the same as those listed above.

The polyester comprising 2,6-naphthalenedicarboxylic acid as the maindicarboxylic acid component is, for example,polyethylene-2,6-naphthalene dicarboxylate. Polyethylene-2,6-naphthalenedicarboxylate may comprise a component other than2,6-naphthalenedicarboxylic acid and ethylene glycol in an amount of 50mol % or less, particularly 20 mol % or less.

Polyethylene terephthalate may be mixed with 50 wt % or less of anotherpolymer, for example, polyethylene-2,6-naphthalate.

Meanwhile, polyethylene-2,6-naphthalene dicarboxylate may be mixed with50 wt % or less of another polymer, for example, polyethyleneterephthalate.

To closely adhere the molten polyester sheet to the surface of thecooling roll in the process of the present invention, static electricityis applied to the molten polyester sheet to closely adhere the sheet tothe cooling roll through the shift of charge, or the pressure of anatmosphere on the side in contact with the cooling roll of the moltenpolyester sheet is made lower than the pressure of an atmosphere on theopposite side to closely adhere the polyester sheet to the cooling roll.

When the former electrostatic adhesion method is employed out of theseand the molten polyester sheet is a molten sheet of a polyestercomprising terephthalic acid as the main acid component, it is desiredthat the resistivity of the sheet be 3×10⁶ to 1×10⁸ Ω.cm and that staticelectricity be applied to the molten polyester sheet to ensure that theamount of initial accumulated charge should be preferably 2.5 to 8.5μC/mm², more preferably 3.0 to 8.0 μC/mm².

When the molten polyester sheet is a molten sheet of a polyestercomprising 2,6-naphthalenedicarboxylic acid as the main acid component,it is desired that the resistivity of the sheet be 1×10⁷ to 5×10⁸ Ω.cmand that static electricity be applied to the molten polyester sheet toensure that the amount of initial accumulated charge should bepreferably 2.2 to 8.0 μC/mm², more preferably 2.5 to 7.5 μC/mm².

When the amount of initial accumulated charge is smaller than the abovelower limit value, it is difficult to obtain a sheet at a high speed bythe electrostatic adhesion method and when the amount is larger than theabove upper limit value, large amounts of coarse particles and foreignmatter are contained in the film or the thermal stability of the polymerbecomes insufficient with the result that the film is readily coloredyellow.

A polyester whose initial accumulated charge is in the range of thepresent invention in a molten state can be prepared by mixing at leastone of an alkali metal, alkali earth metal, Mg, P or compound thereofwith the above polyester. Out of these metal compounds, an Mg compound(for example, magnesium acetate) is preferably used, particularlypreferably used in conjunction with a P compound.

The total amount of the metal compounds is preferably 20 to 2,000 ppm,more preferably 50 to 1,000 ppm, particularly preferably 100 to 600 ppmin terms of metal atoms contained in the polyester.

In the present invention, the molding speed of the polyester sheet ispreferably 65 to 250 m/min in the case of a polyester comprisingterephthalic acid as the main acid component and 40 to 200 m/min in thecase of a polyester comprising 2,6-naphthalenedicarboxylic acid as themain acid component.

The thickness of the molded sheet in the present invention is preferably5 to 400 μm.

According to the present invention, there is also provided a process forproducing a biaxially oriented polyester film by biaxially orienting thepolyester sheet obtained by the above process of the present inventionin a longitudinal direction and a transverse direction. By this process,the transfer of the micro-cracks to the surface of the biaxiallyoriented film can be greatly suppressed.

The biaxial orientation in the present invention is sequential biaxialorientation that an unstretched sheet is preheated and stretched in alongitudinal direction and then in a transverse direction, orsimultaneous biaxial orientation that an unstretched sheet is stretchedin longitudinal and transverse directions simultaneously. Particularlyin the case of sequential biaxial orientation, various known stretchingmethods, for example, a stretching method proposed by JP-A 54-8672 andJP-A 5-177702 may be advantageously employed. For example, anunstretched sheet is heated and stretched in a longitudinal directionrepeatedly at multiple sections to a total draw ratio of 2 to 10 timesand then in a transverse direction to a total draw ratio of 2 to 10times during the step of stretching in the longitudinal direction atmultiple sections and/or after the step of stretching in thelongitudinal direction to achieve a total draw ratio in the bothdirections of 4 to 50 times, preferably 9 to 40 times, particularlypreferably 12 to 30 times.

EXAMPLES

The following examples are given to further illustrate the presentinvention. The present invention is not limited to the followingexamples without departing from the scope of the invention.Characteristic property values were measured by the following methods.

“Part” in examples means “parts by weight”. The longitudinal directionof the film means an extrusion direction in the production of a film andthe transverse direction means a direction perpendicular to thelongitudinal direction on the film plane.

(1) Melting Point of Polyester (Tm)

About 10 mg of a sample is enclosed in an aluminum pan for measurement.The aluminum pan enclosing the sample is set in a differentialthermometer (V4. OB2000 DSC of DuPont Co., Ltd.), heated from 25° C. to300° C. at a rate of 20° C./min, maintained at 300° C. for 5 minutes,taken out from the differential thermometer and placed on ice to bequenched immediately. This pan is then set in the differentialthermometer again to take the melting peak temperature of the polyesterwhich appears when the temperature is elevated from 25° C. at a rate of20° C./min as the melting point (Tm: ° C.) of the polyester.

(2) Falling Temperature Crystallization Temperature (Tc) of Polyester

About 10 mg of a sample is enclosed in an aluminum pan for measurement.The aluminum pan enclosing the sample is set in a differentialthermometer (V4. OB2000 DSC of DuPont Co., Ltd.), heated from 25° C. to300° C. at a rate of 20° C./min, maintained at 300° C. for 5 minutes andcooled at a rate of 10° C./min to take the crystallization peaktemperature generating while cooling as the falling temperaturecrystallization temperature (Tc: ° C.) of the polyester.

(3) Glass Transition Temperature (Tg) of Polyester

About 10 mg of a sample is enclosed in an aluminum pan for measurement.The aluminum pan enclosing the sample is set in a differentialthermometer (V4. OB2000 DSC of DuPont Co., Ltd.), heated from 25° C. to300° C. at a rate of 20° C./min, maintained at 300° C. for 5 minutes,taken out from the differential thermometer and placed on ice to bequenched immediately. This pan is then set in the differentialthermometer again to measure the glass transition temperature (Tg: ° C.)of the polyester by elevating the temperature from 25° C. at a rate of10° C./min.

(4) Surface Temperature (T) of Molten Polyester Sheet

The surface temperature of a center portion in a width direction of themolten polyester sheet 10 mm below the nozzle of an orifice is measuredwith a non-contact thermometer (IT2-60 handy thermometer equipped with apoint marker of Keyence Co., Ltd.: emissitivity set value of 0.93). Theside not in contact with the cooling roll of the molten polyester sheetis measured.

(5) Surface Temperature of Cooling Roll

The surface temperature of the cooling roll before the molten polyestersheet contacts the cooling roll is measured with a non-contactthermometer (TLR-1 roll temperature measuring instrument of TeijinEngineering Co., Ltd.). The emissitivity is set equal to the measurementvalue of a contact type thermometer. The measurement position of thesurface temperature of the cooling roll is within a section between 50mm and 200 mm away from a position before the molten polyester sheetcontacts the cooling roll.

(6) Ventilation Resistance by Vacuum Leakage Method

The ventilation resistance in the present invention means a timerequired for a reduction in the degree of vacuum from a certain value toanother certain value caused by an inflow of air through the grooves ofthe rough surface after a vacuum region with the certain degree ofvacuum is formed in the rough surface (front surface of the coolingroll) by vacuum suction and vacuum suction is stopped.

As for a specific method of measuring ventilation resistance, as shownin the schematic diagram of FIG. 1, a vacuum pump 14 is connected to oneend of a vessel 11 equipped with a vacuum gauge 12 through a vacuum cock13 and a rubber sucker 16 (for example, FPM. PFYK-40 of Myotoku Co.,Ltd.) is attached to the other end by a vacuum hose 15. The effectivevolume from the vacuum cock 13 to the sucker 16 is 100 cc. As shown inthe enlarged sectional view of the sucker portion of FIG. 2, a 40mm-diameter sucker (16, 22) is pressed against the surface 24 of thecooling roll and a 30 mm-diameter porous sheet 23 (for example, 8-L-500Naslon low-density sintered material of Nippon Seisen Co., Ltd.) isplaced and pressed against the center of the sucker so that it contactsthe outer surface of the sucker. When the inside of the 100 cc vessel isevacuated to −93.1 kPa (−700 mmHg) or less by the vacuum pump 14 and thecock 13 is closed, air flows into the vacuum system through the groovesof the rough surface surrounded by the sucker, resulting in a reductionin the degree of vacuum. The time required for a reduction in the degreeof vacuum from −93.1 kPa to −86.45 kPa (−700 mmHg to −650 mmHg) isdefined as ventilation resistance. Before the measurement of ventilationresistance, it is confirmed that the ventilation resistance of apolished glass plate is 20,000 sec or more to check the vacuum leakageof the measuring instrument.

As for the measurement of ventilation resistance, the ventilationresistance of the cooling roll is obtained by measuring 4 locations of acenter portion in a width direction of the cooling roll at a pitch of90° in a revolution direction and averaging the measurement values.

(7) Number of Intersecting Micro-Cracks

A 10 mm long scanning line is drawn on the surface of the cooling roll,observed through an optical microscope (RMP Roll Scope of Union KogakuCo., Ltd.) at a magnification of X100 and photomicrographed. The totalnumber of cracks intersecting the 10 mm long scanning line is obtainedfrom the obtained photomicrograph.

(8) Average Opening Width of Micro-Cracks

A 10 mm long scanning line is drawn on the surface of the cooling roll,observed through an optical microscope (RMP Roll Scope of Union KogakuCo., Ltd.) at a magnification of X500 and photomicrographed. The widthsof all the cracks intersecting the 10 mm scanning line are obtained fromthe obtained photomicrograph and the average of the measurement valuesis taken as average opening width.

(9) Melt Viscosity of Polymer

The molten polymer is maintained in a measurement cylinder at 300° C.for 60 seconds by a Koka type flow tester (of Shimadzu Corporation) inaccordance with JIS K7210 and discharged from a 10 mm-long and 1mm-diameter nozzle under a load of 30 MPa to measure the melt viscosity(Pa.s) of the polymer.

(10) Resistivity of Molten Polymer

The resistivity of the molten polymer is measured in accordance with amethod described in Brit. J. Appl. Phys. vol. 17, pp. 1149 to 1154,1966. The melting temperature of the polymer is 290° C. when the polymeris a polyester comprising terephthalic acid as the main acid componentand 295° C. when the polymer is a polyester comprising2,6-naphthalenedicarboxylic acid as the main acid component, and thevalue of resistivity is measured right after a DC voltage of 1,000 V isapplied.

(11) Amount of Initial Accumulated Charge

The amount of initial accumulated charge in a molten state is measuredin accordance with a method disclosed by JP-A 62-189133. The meltingtemperature of the polymer is 275° C. when the polymer is a polyestercomprising terephthalic acid as the main acid component and 295° C. whenthe polymer is a polyester comprising 2,6-naphthalenedicarboxylic acidas the main acid component, and the amount of initial accumulated chargeis calculated from voltage and current values when a DC voltage of 1,200V is applied for 3 minutes.

(12) Thickness of Sheet or Film

10 locations of a sheet or film are measured with a micrometer and theaverage of the measurement values is taken as the thickness of the sheetor film.

(13) Amount of Metal Compound

The amount (ppm) of a metal contained in a polymer is measured withfluorescent X-radiation (the 3270 fluorescent X-radiation of RigakuDenki Kogyo Co., Ltd.).

Example 1-1

Polyethylene-2,6-naphthalene dicarboxylate (Tm: 270° C., Tc: 220° C.)was extruded from a T die having a nozzle end temperature of 255° C.into a 150 μm-thick molten polyester sheet which was then closelyadhered to and cooled by a rotary cooling drum having a micro-crackedsurface whose surface temperature was maintained at 35° C. by theelectrostatic adhesion method to form a polyester sheet at a rate of 53m/min. The surface temperature of the extruded sheet product about 10 mmbelow the nozzle was 262° C. The used cooling roll had an averagemicro-crack groove width on the surface of 2.3 μm and an averageventilation resistance before film formation of 380 sec, and the maximumcasting speed at which a defect-free satisfactory sheet evaluated at thebeginning of film formation could be obtained was 56 m/min.

The obtained polyester sheet was stretched to 3.9 times in alongitudinal direction and then 4.2 times in a transverse directionsequentially. Under the above conditions, biaxially oriented polyesterfilms were formed for 6 days. On the fourth day after the start of filmformation, film formation was suspended to remove the sublimateaccumulated on the surface of the cooling roll (the surface of the rollwas wiped with cloth impregnated with an aqueous solution containing adetergent and wrung hard) and then continued for another two days. Filmscould be formed smoothly without a trouble such as a break. The obtainedbiaxially oriented films were transparent and smooth high-quality filmshaving no surface defect. When the cooling roll was evaluated after theend of film formation, there was no time change in the maximum castingspeed (56 m/min) as compared with the beginning of film formation andthe ventilation resistance remained almost unchanged at 383 sec.

Comparative Example 1-1

Film formation was carried out for 6 days by the same method under thesame conditions as in Example 1-1 except that the nozzle end temperaturewas changed to 307° C., the surface temperature of the extruded sheetproduct was 312° C., and the frequency of the operation of removing thesublimate accumulated on the surface of the cooling roll was changed.Since the accumulation speed of the sublimate on the surface of thecooling roll was fast, the sublimate accumulated on the surface of thecooling roll had to be removed three times in the course of filmformation in order to prevent the occurrence of an orange skin-likedefect on the film. Since a trouble that the film was frequently brokenin the step of stretching in a transverse direction occurred on thesixth day from the start of film formation, film formation was stopped.A time change in the maximum casting speed was observed as the maximumcasting speed evaluated at the end of film formation was 52 m/mincompared with the maximum casting speed (56 m/min) evaluated at thebeginning of film formation. Deterioration in performance was seenbecause the average ventilation resistance at the end of film formationwas 391 sec, compared with 380 sec before film formation. There was aportion having a low ventilation resistance of 436 sec.

Example 1-2

After the cooling roll used for 6 days of film formation in ComparativeExample 1-1 was regenerated and cleaned, its average ventilationresistance and maximum casting speed were restored to initial values of380 sec and 56 m/min at the beginning of film formation, respectively.Using this cooling roll, biaxially oriented polyester films were formedfor 6 days under the same conditions as in Example 1-1. The same resultsas in Example 1-1 were obtained and no break trouble occurred.

Example 1-3

Biaxially oriented polyester films were formed for 6 days under the sameconditions as in Example 1-2 using a cooling roll which was used to formfilms for 6 days in Example 1-2 and from which the sublimate accumulatedon the surface was removed (the cooling roll was not regenerated andcleaned). The same results as in Example 1-2 were obtained.

Example 2-1

Polyethylene terephthalate (Tm: 260° C., Tg: 79° C., Tc: 160° C.) wasextruded from a T die (nozzle having a width of 400 nm) having a nozzleend temperature of 260° C. into a 210 μm-thick polyester sheet which wasthen closely adhered to and cooled by a rotary cooling drum having amicro-cracked surface whose surface temperature was maintained at 30° C.by the electrostatic adhesion method to form a polyester sheet at a rateof 65 m/min. The surface temperature of the extruded sheet product about10 mm below the nozzle was 265° C. Subsequently, this unstretched sheetwas stretched to 3.6 times in a longitudinal direction and then 3.9times in a transverse direction and heat set at 215° C. to obtain abiaxially oriented film.

The cooling roll had a micro-cracked surface with a ventilationresistance measured by the vacuum leakage method of 29 sec as average(the average number of cracks intersecting a 10 mm scanning line on thesurface was 75 and the average opening width of all the cracksintersecting the 10 mm scanning line was 12 μm). During casting, thesurface temperature of the cooling roll 100 mm at an upstream from thepoint where the sheet product landed (measured with the TLR-1 rolltemperature measuring instrument) was 52° C.

A very slight micro-crack transferred pattern was observed on theobtained unstretched sheet but an orange skin-like defect was notobserved on the biaxially oriented film. The surface smoothness of thefilm satisfied quality standards for a base material for video magneticrecording materials. The height of the mountain range of the transferredpattern of the unstretched sheet was about 0.06 μm but the height of thetransferred pattern of the biaxially oriented film could not be detectedwith a detection accuracy of 0.02 μm.

Example 3-1

Polyethylene-2,6-naphthalene dicarboxylate (Tm: 270° C., Tg: 121° C.)having a melt viscosity of 1,200 Pa.s was molten and extruded from a 400mm-wide nozzle into a 120 μm-thick sheet form which was then cast with acooling roll at a rate of 55 m/min using the electrostatic adhesionmethod to obtain a similar unstretched sheet to that of Example 1-1except that the following cooling roll was used. Subsequently, thisunstretched sheet was stretched to 3.9 times in a longitudinal directionand then 4.1 times in a transverse direction and heat set at 225° C. toobtain a biaxially oriented film.

The cooling roll had a micro-cracked surface with an average ventilationresistance measured by the vacuum leakage method of 29 sec (the numberof cracks intersecting a 10 mm scanning line on the surface was 75 andthe average opening width of all the cracks intersecting the 10 mmscanning line was 12 μm). The surface temperature of the cooling roll100 mm at an upstream from the point where the sheet product landed was55° C. during casting (measured with the TLR-1 roll temperaturemeasuring instrument).

A very slight micro-crack transferred pattern was seen on the obtainedunstretched sheet but an orange skin-like defect was not seen on thebiaxially oriented film and the surface smoothness of the film satisfiedquality standards for a base material for video magnetic recordingmaterials. The height of the mountain range of the transferred patternof the unstretched sheet was about 0.05 μm but the height of thetransferred pattern of the biaxially oriented film could not be detectedwith a detection accuracy of 0.01 μm.

Example 4-1

An unstretched sheet was obtained in the same manner as in Example 1-1except that polyethylene-2,6-naphthalene dicarboxylate (Tg: 121° C., Tm:270° C.) containing a magnesium compound (80 ppm), a phosphorus compound(30 ppm) and an antimony compound (250 ppm) and having a resistivity atthe time of melting of 4×10⁷ Ω.cm was molten in an extruder and extrudedfrom a nozzle into a 180 μm-thick sheet form which was then closelyadhered to a cooling roll having a micro-cracked surface described belowby the electrostatic adhesion method to be solidified. The surfacetemperature of the cooling roll near the landing point of the moltensheet was 63° C. and the maximum sheet molding speed was 98 m/min. Thisunstretched sheet was wound up and then stretched to 3.6 times in alongitudinal direction and 3.9 times in a transverse direction.

The cooling roll used for casting had a micro-cracked surface, theaverage ventilation resistance on the surface measured by the vacuumleakage method thereof was 65 sec, the average number of cracksintersecting a 10 mm scanning line on the surface was 105, and theaverage opening width of all the cracks intersecting the 10 mm scanningline was 7 μm.

A very slight micro-crack transferred pattern was seen on the obtainedmolded sheet but an orange skin-like defect was not seen on thebiaxially oriented film, there was no coarse foreign matter, and thesurface smoothness of the film satisfied quality standards for a basematerial for video magnetic recording materials.

Example 5-1

An unstretched sheet was obtained in the same manner as in Example 2-1except that polyethylene terephthalate (Tg: 79° C., Tm: 260° C.)containing a magnesium compound (90 ppm), a phosphorus compound (30 ppm)and an antimony compound (330 ppm) and having a resistivity at the timeof melting of 7×10⁶ Ω.cm was extruded from a nozzle into a 180 μm-thicksheet form which was then closely adhered to a cooling roll having amicro-cracked surface described below by the electrostatic adhesionmethod to be solidified. The surface temperature of the cooling rollnear the landing point of the molten sheet was 48° C., and the maximumsheet molding speed was 130 m/min. This unstretched sheet was wound up,stretched to 3.6 times in a longitudinal direction and 3.9 times in atransverse direction, and heat set at 215° C.

The cooling roll used for casting had a micro-cracked surface, theaverage ventilation resistance on the surface measured by the vacuumleakage method thereof was 65 sec, the average number of cracksintersecting a 10 mm scanning line on the surface was 105, and theaverage opening width of all the cracks intersecting the 10 mm scanningline was 7 μm.

A very slight micro-crack transferred pattern was observed on theobtained molded sheet but an orange skin-like defect was not seen on thebiaxially oriented film, there was no coarse foreign matter, and thesurface smoothness of the film satisfied quality standards for a basematerial for video magnetic recording materials.

Example 6-1

The cooling roll used for casting had a micro-cracked surface, theaverage ventilation resistance on the surface measured by the vacuumleakage method thereof was 65 sec, the average number of cracksintersecting a 10 mm scanning line on the surface was 640, and theaverage opening width of all the cracks intersecting the 10 mm scanningline was 6 μm.

The tape-like discharge electrode was a stainless steel tape having arectangular section, a thickness of 50 μm and a width of 8 mm anduniform in shape in a lengthwise direction, and a positive DC voltage of6.7 kV was applied to the electrode.

An unstretched sheet was formed in the same manner as in Example 2-1except that polyethylene terephthalate was extruded from a nozzle into asheet form having a thickness of 180 μm and a width of 420 mm which wasthen electrostatically closely adhered to the above cooling roll havinga micro-cracked surface by the above tape-like discharge electrode(installed at a position 2 mm away from the surface of the molten sheetin such a manner that the long axis of the section of the tape-likeelectrode became almost perpendicular to the surface of the cooling rollnear the position where the molten sheet landed on the surface of thecooling roll) to be cast, and rolled up.

At a casting speed of 100 m/min, the allowable traveling width of thetape-like electrode for the stable production of a defect-freehigh-quality sheet was 1.5 mm in the circumferential direction of thecooling roll, was very stable and remained unchanged after the passageof 24 hours. The sheet had excellent surface smoothness without anorange skin-like defect. The sheet which was cast and rolled up wasstretched to 3.6 times in a longitudinal direction and 3.9 times in atransverse direction and heat set at 215° C. An orange skin-like defectwas not seen on the obtained biaxially oriented film and the surfacesmoothness of the film satisfied quality standards for a base materialfor high-grade video magnetic recording materials.

Example 7-1

The cooling roll used for casting had a micro-cracked surface, theaverage ventilation resistance on the surface measured by the vacuumleakage method thereof was 75 sec, the average number of cracksintersecting a 10 mm scanning line on the surface was 610, the averageopening width of all the cracks intersecting the 10 mm scanning line was4 μm, and the surface roughness (Ra) was 0.09 μm.

Polyethylene terephthalate was extruded from a nozzle into a 210μm-thick sheet form which was then closely adhered to the above coolingroll having a micro-cracked surface to be cast. An unstretched sheet wasformed in the same manner as in Example 2-1 except that a vacuum chamberwas interposed between the die and the cooling roll to create a vacuumatmosphere near the surface in contact with the cooling roll of theextruded sheet by means of decreasing pressure in the chamber in orderto closely adhere the extruded sheet to the cooling roll, and rolled up.

When the vacuum degree of the vacuum chamber was maintained at 1,400 Pa,the maximum speed at which a cooled sheet could be produced stablywithout generating air bubbles by the caught air was 78 m/min. Thismaximum speed was stable and remained unchanged after the passage of 24hours. An orange skin-like defect was not observed on the sheet and thesurface smoothness of the sheet was satisfactory. The rolled sheet wasstretched to 3.6 times in a longitudinal direction and 3.9 times in atransverse direction and heat set at 215° C. An orange skin-like defectwas not seen on the obtained biaxially oriented film and the surfacesmoothness of the film satisfied quality standards for a base materialfor high-grade video magnetic recording materials.

Comparative Example 7-1

Film formation was carried out using the same device and method as inExample 1-1 except that a cooling roll having a satin finished surfacewith a surface roughness (Ra) of 0.41 μm was used. As a result, themaximum casting speed was 56 m/min. An orange skin-like defect wasobserved on the obtained sheet. An orange skin-like defect was also seenon a biaxially oriented film obtained from this sheet and this film didnot satisfy quality standards for a base material for high-grade videomagnetic recording materials. This film was inferior in casting speedand film quality to a film formed by using a cooling roll having amicro-cracked surface.

Comparative Example 7-2

Film formation was carried out using the same device and method as inExample 1-1 except that a cooling roll having grinding streaks with asurface roughness (Ra) of 0.15 μm was used. As a result, the maximumcasting speed was 44 m/min which was much lower than a cooling rollhaving a micro-cracked surface.

Air bubbles formed by the caught air became more marked along thepassage of film formation time and the casting speed at which castingcould be carried out stably without an air bubble defect was reduced to41 m/min after 12 hours.

A conventional cooling roll having small roughness had a small speedincreasing effect and experienced a great time change.

Example 8-1

An unstretched sheet was obtained in the same manner as in Example 2-1except that polyethylene terephthalate was molten and extruded from anozzle into a 250 μm-thick sheet form which was then cast with a coolingroll having a micro-cracked surface shown below using the electrostaticadhesion method. Subsequently, this unstretched sheet was stretched to 2times at 110° C., 1.1 times at 120° C. and 3 times at 110° C. with alongitudinal-direction multi-stage stretching machine at a total drawratio of 6.6 times. Then, the film was further stretched to 4.3 times ina transverse direction and heat set at 215° C. to obtain a biaxiallyoriented film. The total draw ratio in longitudinal and transversedirections was 28 times.

The cooling roll had a micro-cracked surface with an average ventilationresistance measured by the vacuum leakage method of 125 sec (the averagenumber of cracks intersecting a 10 mm scanning line on the surface was150 and the average opening width of all the cracks intersecting the 10mm scanning line was 6 μm).

An orange skin-like defect was not observed on the obtained biaxiallyoriented film and the surface smoothness of the film satisfied qualitystandards for a base material for high-grade video magnetic recordingmaterials.

Example 8-2

A biaxially oriented film was obtained in the same manner as in Example1 except that a 190 μm-thick unstretched sheet was molded with the samecasting device as in Example 8-1 and stretched to 2.2 times in alongitudinal direction at 100° C., 4 times in a transverse direction at140° C. and then 2.3 times in a longitudinal direction again at 160° C.The total draw ratio in longitudinal and transverse directions was 20times.

An orange skin-like defect was not seen on the obtained biaxiallyoriented film and the surface smoothness of the film satisfied qualitystandards for a base material for high-grade video magnetic recordingmaterials.

Example 9-1

An unstretched sheet was obtained in the same manner as in Example 2-1except that polyethylene terephthalate was molten and extruded from anozzle into a 240 μm-thick sheet form which was then cast with a coolingroll having a micro-cracked surface shown below at a rate of 100 m/minusing the electrostatic adhesion method. Subsequently, this unstretchedsheet was stretched to 2.2 times at 110° C., 1.05 times at 120° C. and 4times at 90° C. in a longitudinal direction with alongitudinal-direction multi-stage stretching machine and then 4.0 timesin a transverse direction and heat set at 215° C. to obtain a biaxiallyoriented film. The total draw ratio in the longitudinal direction was9.24 times and the wind-up speed was 924 m/min.

The cooling roll had a micro-cracked surface with an average ventilationresistance measured by the vacuum leakage method of 115 sec (the averagenumber of cracks intersecting a 10 mm scanning line on the surface was180 and the average opening width of all the cracks intersecting the 10mm scanning line was 6 μm).

An orange skin-like defect was not seen on the obtained biaxiallyoriented film and the surface smoothness of the film satisfied qualitystandards for a base material for high-grade video magnetic recordingmaterials.

Example 9-2

A biaxially oriented film was obtained in the same manner as in Example1 except that a 220 μm-thick unstretched sheet was cast with the samecasting device as in Example 9-1 at a rate of 120 m/min, stretched to2.2 times in a longitudinal direction at 100° C., 3.6 times in atransverse direction at 140° C. and 2.7 times in the longitudinaldirection again at 170° C., and wound up at a rate of 708 m/min. Thetotal draw ratio in the longitudinal direction was 5.9 times.

An orange skin-like defect was not seen on the obtained biaxiallyoriented film and the surface smoothness of the film satisfied qualitystandards for a base material for high-grade video magnetic recordingmaterials.

Example 10-1

An unstretched sheet was obtained in the same manner as in Example 2-1except that polyethylene terephthalate (Tg: 79° C., Tm: 260° C.)containing a magnesium compound (100 ppm), a phosphorus compound (40ppm) and an antimony compound (300 ppm) and having an initialaccumulated charge at the time of melting of 5.9 μC/mm² was molten in anextruder and extruded from a nozzle into a 180 μm-thick sheet form whichwas then closely adhered to a cooling roll having a micro-crackedsurface shown below using the electrostatic adhesion method to besolidified. The surface temperature of the cooling roll near the landingpoint of the molten sheet was 48° C. and the maximum sheet molding speedwas 122 m/min. This unstretched sheet was wound up, stretched to 3.6times in a longitudinal direction and 3.9 times in a transversedirection and heat set at 215° C.

The cooling roll used for casting had a micro-cracked surface, theaverage ventilation resistance on the surface measured by the vacuumleakage method thereof was 65 sec, the average number of cracksintersecting a 10 mm scanning line on the surface was 105, and theaverage opening width of all the cracks intersecting the 10 mm scanningline was 7 μm.

A very slight micro-crack transferred pattern was observed on theobtained molded sheet but an orange skin-like defect was not seen on thebiaxially oriented film, there was no coarse foreign matter, and thesurface smoothness of the film satisfied quality standards for a basematerial for video magnetic recording materials.

Example 11-1

An unstretched sheet was obtained in the same manner as in Example 1-1except that polyethylene-2,6-naphthalate (Tg: 121° C., Tm: 270° C.)containing a magnesium compound (90 ppm), a phosphorus compound (40 ppm)and an antimony compound (280 ppm) and having an initial accumulatedcharge at the time of melting of 5.5 μC/mm² was molten in an extruderand extruded from a nozzle into a 160 μm-thick sheet form which was thenclosely adhered to a cooling roll having a micro-cracked surface shownbelow using the electrostatic adhesion method to be solidified. Thesurface temperature of the cooling roll near the landing point of themolten sheet was 63° C., and the maximum sheet molding speed was 88m/min. This unstretched sheet was wound up, stretched to 3.6 times in alongitudinal direction and 3.9 times in a transverse direction and heatset at 225° C.

The cooling roll used for casting had a micro-cracked surface, theaverage ventilation resistance on the surface measured by the vacuumleakage method thereof was 70 sec, the average number of cracksintersecting a 10 mm scanning line on the surface was 105, and theaverage opening width of all the cracks intersecting the 10 mm scanningline was 7 μm.

A very slight micro-crack transferred pattern was observed on theobtained molded sheet but an orange skin-like defect was not seen on thebiaxially oriented film, there was no coarse foreign matter, and thesurface smoothness of the film satisfied quality standards for a basematerial for video magnetic recording materials.

Example 12-1

A polyester sheet was formed by extruding polyethylene terephthalate(Tm: 260° C., Tg: 79° C.) from a T die having a nozzle end temperatureof 280° C. into a 200 μm-thick molten polyester sheet form, closelyadhering the sheet to a rotary cooling drum having a micro-crackedsurface whose surface temperature was maintained at 25° C. by theelectrostatic adhesion method and gradually increasing the casting speedfrom 50 m/min while cooling the sheet. The surface temperature of theextruded sheet product about 10 mm below the nozzle was 280° C.

The total area Ac of the micro-cracked portion of the surface of thecooling roll used was 0.051 mm², 0.052 mm², 0.048 mm² and 0.049 mm²(average value was 0.050 mm²) at four locations crossing one another at90° in a circumferential direction of a center portion in a widthdirection of the roll based on 1 mm² of the surface in contact with themolten polyester sheet of the cooling roll. The cooling roll had adiameter of 2.0 m, a width of 1.0 m and a shell thickness of 15 mm. Themaximum casting speed was 180 m/min and the obtained polyester sheet hadno defect and was excellent. The temperature (Ti) of cooling water to beintroduced into the inside of the cooling roll was 24° C., thetemperature (To) of cooling water to be discharged to the outside of thecooling roll was 27° C., and the temperature difference of cooling water(To−Ti) was 3° C.

The obtained polyester sheet was then stretched to 3.4 times in alongitudinal direction and 4.0 times in a transverse directionsequentially. Biaxially oriented polyester films could be formed underthe above conditions for 7 days. Film formation could be performedwithout a trouble such as a break during this period. The obtainedbiaxially oriented films were transparent and smooth high-quality filmshaving no surface defect.

Examples 12-2 to 12-4 and Comparative Example 12-1

Polyester sheets and biaxially oriented polyester films were produced inthe same manner as in Example 12-1 except that the type of the polyesterused, the total area Ac of the micro-cracked portion of the surface ofthe cooling roll, the diameter of the cooling roll, the thickness of theshell of the cooling roll, the temperature (Ti) of cooling water to beintroduced into the inside of the cooling roll, the temperature (To) ofcooling water to be discharged to the outside of the cooling roll andthe temperature difference (To−Ti) of cooling water were changed asshown in Table 1.

The maximum casting speed, the qualities of the obtained polyestersheets and the obtained biaxially oriented polyester films, thecontinuous film formation period and film formation conditions duringthe above period are shown in Table 1. In film formation using themirror surface cooling roll of Comparative Example 12-1, the moldingspeed could not be increased due to the surface defect of the sheet.

In the column for the type of polyester in Table 1, PET stands forpolyethylene terephthalate (Tm: 260° C., Tg: 79° C.) and PEN stands forpolyethylene-2,6-naphthalene dicarboxylate (Tm: 270° C., Tg: 121° C.).

TABLE 1 cooling roll temperture of cooling water total area shell inputoutput temperature thickness continuous type of of cracks diameterthickness temperature temperature difference of molded molding speedoperation time polyester (mm²) (m) (mm) (° C.) (° C.) (° C.) sheet (μm)(m/min) (day) Ex. 12-1 PET 0.05 2.0 10 24 27 3 140 180 7 Ex. 12-2 PET0.20 0.8 7 24 27 3 30 120 10 Ex. 12-3 PET 0.02 3.0 20 20 24 4 350 80 6Ex. 12-4 PEN 0.05 2.0 10 40 43 3 120 150 7 C.Ex. 12-1 PET mirror 2.0 1024 26 2 140 60 8 surface Ex. = Example C.Ex. = Comparative Example

What is claimed is:
 1. A process for producing a polyester sheet bydropping a molten polyester sheet extruded from an orifice-form nozzleon a cooling roll having the grooves of a large number of micro-cracksformed on the surface, closely adhering it to the cooling roll andsolidifying it on the cooling roll, wherein the surface temperature (T,° C.) of the molten polyester sheet 10 mm below the orifice-form nozzleis maintained at a temperature which satisfies the following expression(1): (Tc+20)° C.≦T≦(Tm+40)° C.  (1) wherein Tc and Tm are the fallingtemperature crystallization temperature (° C.) and melting point (° C.)of the polyester, respectively and T is as defined hereinabove, and thesurface temperature of the cooling roll when it contacts the moltenpolyester sheet is controlled to a range of 5 to (Tg−20)° C. tocontinuously form the polyester sheet while preventing the adhesion of asublimate from the molten polyester to the inside of the groove of eachmicro-crack of the cooling roll.
 2. The process of claim 1, wherein thecooling roll shows a ventilation resistance measured by a vacuum leakagemethod of 10,000 sec or less based on the grooves of the large number ofmicro-cracks formed on the surface of the cooling roll.
 3. The processof claim 1, wherein the cooling roll has the grooves of at least fivemicro-cracks intersecting a 10 mm virtual straight line drawn in anydirection on the surface of the cooling roll, 70% or more of the groovesof the intersecting micro-cracks have a width of 0.1 to 100 μm atintersections with the virtual straight line, and the total width of thegrooves of the micro-cracks intersecting the virtual straight line atall the intersections is 5 mm or less.
 4. The process of claim 1,wherein the total opening area of the grooves of the large number ofmicro-cracks open to the surface of the cooling roll is 0.01 to 0.3 mm²per 1 mm² of the surface of the cooling roll.
 5. The process of claim 1,wherein the cooling roll cools the polyester sheet on the surface byintroducing cooling water into the inside thereof and discharging it anda rise in the temperature of cooling water to be discharged from thetemperature of cooling water to be introduced is controlled to 1 to 10°C.
 6. The process of claim 1, wherein the diameter of the cooling rollis 0.6 to 4.0 m and the thickness of the shell having the grooves of thelarge number of micro-cracks on the surface of the cooling roll is 5 to30 mm.
 7. The process of claim 1, wherein the molten polyester sheet isclosely adhered to the cooling roll through the shift of charge byapplying static electricity to the molten polyester sheet.
 8. Theprocess of claim 7, wherein the molten polyester sheet is a molten sheetof a polyester comprising terephthalic acid as the main acid componentand has a resistivity of 3×10⁶ to 1×10⁸ Ω.cm and static electricity isapplied to the molten polyester sheet to ensure that the amount ofinitial accumulated charge should be 2.5 to 8.5 μC/mm².
 9. The processof claim 8, wherein the polyester sheet is produced at a rate of 65 to250 m/min.
 10. The process of claim 7, wherein the molten polyestersheet is a molten sheet of a polyester comprising2,6-naphthalenedicarboxylic acid as the main acid component and has aresistivity of 1×10⁷ to 5×10⁸ Ω.cm and static electricity is applied tothe molten polyester sheet to ensure that the amount of initialaccumulated charge should be 2.2×8.0 μC/mm².
 11. The process of claim10, wherein the polyester sheet is produced at a rate of 40 to 200m/min.
 12. The process of claim 1, wherein the molten polyester sheet isclosely adhered to the cooling roll by making the pressure of anatmosphere on the side in contact with the cooling roll of the moltenpolyester sheet lower than the pressure of an atmosphere on the oppositeside.
 13. A process for producing a biaxially oriented polyester film,comprising biaxially orienting the polyester sheet obtained by theprocess of claim 1 in a longitudinal direction and a transversedirection.
 14. The process of claim 13, wherein the product of a drawratio in a longitudinal direction and a draw direction in a transversedirection is 4 to 50 times.